The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode, the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
In the case of current-in-plane (CIP) spin valve sensors, the sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. But, in the case of current-perpendicular-to-plane (CPP) spin valve sensors, the non-magnetic, electrically insulating gap layers are absent, electrical connection being made to the sensor through conductive layers located above and below the sensor in place of the former insulating gap layers. In a merged magnetic head, a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head, the second shield layer and the first pole piece layer are separate layers.
The ever increasing demand for increased data rate and data capacity has lead a relentless push to develop magnetoresistive sensors having improved signal amplitude. Sensors that show promise in achieving higher signal amplitude are current perpendicular to plane (CPP) sensors. Such sensors conduct sense current from top to bottom, perpendicular to the planes of the sensor layers. Examples of CPP sensors include current perpendicular to plane giant magnetoresistive sensors (CPP GMR sensors), and tunnel valves. A CPP GMR sensor operates based on the spin dependent scattering of electrons through the sensor, similar to a more traditional CIP GMR sensor except that, as mentioned above, the sense current flows perpendicular to the plane of the layers. A tunnel valve sensor operates based on the spin dependent tunneling of electrons through a thin, non-magnetic, electrically insulating barrier layer.
As sensors become ever smaller, free layer stability becomes a serious issue. As the free layer becomes smaller, it becomes increasingly difficult to keep the magnetization of the free layer biased in a desired direction. In addition, traditional biasing schemes, such as having hard bias layers at either side of the sensor stack are less desirable due to the potential for shorting sense current across the bias layers, and the resulting need to insulate the bias layer from the sensor stack (which reduces the strength of the bias field available for biasing the free layer). An in stack bias structure could be used to replace the traditional hard bias layers, however, such in stack bias structures consume a large amount of gap budget. In stack bias structures (especially the AFM layers used in such structures) are extremely thick and result in a greatly increased gap distance, which is unacceptable in current and future generation read sensors.
Therefore, there is a need for a CPP magnetoresitive sensor design that provides strong pinning, while maintaining a small gap thickness. Such a design would preferably avoid the use of hard bias layers disposed at lateral sides of the sensor. Such a design would also preferably minimize parasitic resistance that would otherwise degrade sensor performance.